Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Moreover, to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radio communication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telecommunications System (UMTS) which is an existing third generation (3G) radio communication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
As such new systems and technologies enter the marketplace, the general trend is to support more users and greater bandwidth in a given geographic area. This gives rise to more and more interference between transmitters and receivers operating in close proximity to one another. FIG. 1 illustrates some basic interference concepts. Therein, a base station (BS) 100 transmits radio signals on the downlink towards user equipments (UEs) 102 and 104 within its cell 106. For example, BS 100 can transmit a signal 108 which is intended for reception on a downlink radio bandwidth resource by UE 102. At the same time, however, BS 100 may transmit a signal 111 intended for reception by UE 104. Some portion of the signal energy associated with signal 111 may interfere with the reception of the signal 104 by UE 102, which type of interference is sometimes referred to as “intracell” interference. Similarly, other base stations associated with neighboring cells may also be transmitting signals which can be received by UE 102 in a manner which overlaps with its reception of signal 108. An example of such “intercell” interference is shown in FIG. 1 as BS 111 transmitting signals 112.
Various techniques have been used historically to combat interference, e.g., orthogonal channelization, interference rejection, etc., in order to increase channel capacity. Downlink interference alignment is a relatively new, joint transmitter-receiver strategy that attempts to align interfering signals. In MIMO networks, for example, interference alignment uses the spatial dimension offered by multiple antennas for alignment. The main concept behind downlink interference alignment techniques is that UEs coordinate their downlink transmissions from the BSs, and the base stations' transmitters use a precoding scheme, such that the downlink interference signals lie in a reduced dimensional subspace at each UE's receiver relative to the desired downlink signal.
However, this technique requires feedback from the UEs in the form of channel state information (CSI) to be transmitted to the network. The transmission of CSI information by the UEs reduces the available bandwidth for the transmission of other data in these systems.